1. Field of the Invention
The invention relates to the detection, the pinpointing and the determination of the velocity of mobile bodies in the vicinity of the ground by a bi-static radar system.
2. Background of the Invention
The document xe2x80x9cMultimode multistatics for passive/active airborne surveillance, Robert F. Ogrodnik, Rome Air Development Center, Griffiss AFB N.Y. 13441-5700, CP381 (Supp) presents the use of existing public telebroadcasting transmissions as transmission signals of a bi-static radar, which thus benefits from extended geographical coverage and absolute discretion. The functions of the detection, pinpointing and determination of the velocity are carried out by deploying a distinct array of receivers, covering the observation zone, associated with centralized processing of the information originating from the backscattering of the signals by a mobile body.
It is possible to evaluate the performance of such a concept by applying the general laws of radar to the current characteristics of analog telebroadcast transmissions. Thus, the presence of line synchronization in the telebroadcast signals leads to a plurality of solutions for distance measurement; likewise, the accuracy of this measurement, inversely proportional to the useful bandwidth of the transmission signals, cannot be other than mediocre.
The advent of digital television has prompted a complete rethink of the nature of the transmission signals, leading to the definition of a COFDM (Coded Orthogonal Frequency Division Multiplex) standard, exhibiting numerous technical advantages, elimination of disturbing echoes for example, and commercial advantages, a large number of accessible programs.
The present invention aims to utilize, in the field of radar, such digital signals.
Accordingly, it uses telebroadcast terrestrial transmissions, which commonly operate on the basis of a plurality of fixed frequency carriers, afforded a symbol-based digital modulation, processed by carrier-wise orthogonal frequency multiplexing.
A receiver for detecting mobile objects which uses such radio transmitters, may be regarded as known (xe2x80x9cLe radar muetxe2x80x9d [The silent radar], B. CARRARA, P. TOURTIER, M. PECOT; Radar Colloquium 1994, Palais des Congrxc3xa8s, Paris) and which comprises:
radioelectric reception means, for picking up the signals backscattered by objects under the effect of said radio transmitters,
processing means for performing a discrimination of such objects according to distance/Doppler bins, and
auxiliary means for acquiring a synchronization reference with respect to the transmitters, so as to clamp these distance/Doppler bins in time.
The invention proposes a receiver, characterized in that it comprises (auxiliary) means for receiving and demodulating the direct radio telebroadcast signal received from one at least of the transmitters, so as to determine their instants of synchronization and their symbols, and in that the processing means comprise:
Doppler distribution means, able to apportion the signals received according to a plurality of Doppler pathways, while affording them in each pathway a frequency shift associated with the rank of this Doppler pathway,
distance compression means, able to form the sum, for all the carriers, of the correlations, in homologous forms, of the output of each Doppler pathway with the symbols available at the output of the auxiliary means, thereby yielding samples arranged in distance/Doppler bins,
means of coherent Doppler integration over a chosen time, thereby yielding integrated distance/Doppler bins, and
means of postprocessing with tracking, devised so as to contrast the echoes obtained in these bins, so as to obtain radar plots containing position/velocity information on one or more objects.
Preferably, the receiver includes, at the output of its radioelectric reception stage, an analog/digital converter, followed by a HILBERT transform stage, so as to obtain amplitude/phase complex digital signals. The Doppler distribution means and the subsequent stages operate digitally.
Very advantageously, the distance compression means comprise Fourier transformation stages on sliding temporal data trains arising from the Doppler distribution, thereby regenerating the symbols as received, then temporal correlators thereof with the output of the auxiliary means, and a summator of the output of the correlators for the set of carriers.
It is currently preferred that the radioelectric reception means comprise an array antenna, and that the processing means operate selectively on pathways respectively associated with distinct antenna elements or groups of antenna elements, up to the postprocessing means, which at the head end comprise means of recombination of the outputs of these pathways, with a view to beam formation. Preferably, the beam recombination means operate adaptively, as a function of the mobile bodies tracked and the echoes observed.
According to another aspect of the invention, the postprocessing means comprise a multistatic processing stage, for defining each radar plot on the basis of several transmitter/receiver pairs.
The invention also envisages a system for detecting mobile objects, which comprises several receivers sited at chosen places, and means for contrasting the radar plots which these receivers yield.
According to a beneficial adjunct, in addition to the telebroadcasting transmitters proper, the system comprises additional transmitters sited at chosen places and operating with the same modulation as the telebroadcasting transmitters proper.
Another variant consists in supplementing the system with means for addressing service information to the telebroadcasting transmitters, for the purposes of retransmission in their modulation.